As defined by the FCC, an ultra-wideband (UWB) signal is an antenna transmission in the range of 3.1 GHz up to 10.6 GHz at a limited transmit power of −41.3 dBm/MHz with an emitted signal bandwidth that exceeds the lesser of 500 MHz or 20% of the center frequency. UWB signals are currently employed for high-bandwidth, short range communications that use high bandwidth radio energy that is pulsed at specific time instants.
Applications for FCC-defined UWB transmissions include distance-based location and tracking applications, and localization techniques that employ precision time-of-arrival measurements. Examples of such UWB applications include radio frequency identification (RFID) tags that employ UWB communication technology for tracking, localization and transmitting information. Other types of UWB applications include precision radar imaging technology. Inventory tracking has been implemented through the use of passive, active and semi-passive RFID devices. These devices have widespread use, and typically respond to interrogation or send data at fixed intervals.
A high density active radio frequency identification (aRFID) environment can easily exceed 1000 aRFID tags for certain application installations, such as cattle feedlot applications where individual cows are each tagged with an aRFID tag. Currently, aRFID installations such as these may be implemented using a maximum of approximately 1000 aRFID tags per each RFID receiver that is provided for the installation. However, aRFID environments may routinely contain in excess of 40,000 tags within a 1 to 2 sq mile area. One previous attempt that has been made to reliably receive and process tag data, and to perform geolocation calculations in such environments, is to use software-only coding schemes in order to help distinguish between multiple tags. This method typically works up to the point where available bandwidth is exceeded due to the number of bits being transmitted (˜100 bits per tag transmission) and the number of tags in the environment (˜1000). Existing RFID tag geolocation technologies employ RFID tags which typically report data at a fixed rate, which is acceptable for low tag density environments (i.e., tag density less than approximately 1000) where interleaved and colliding packets are not problematic.
Traditional time difference of arrival (TDOA) techniques that are employed to locate emitters, such as transmitting RFID tags, require that the absolute time of arrival (TOA) of an emitted signal at each of two or more receivers be recorded and the difference taken, or require that the two signals be processed using a cross correlation method. The primary source of error in determining the absolute TOA is the accuracy with which the arrival time of the emitted signal may be measured at each receiver. Although a high degree of timing accuracy can, in principal, be obtained by employing highly synchronized clocks at each receiver (e.g., using synchronized atomic clocks), this can be a very expensive option. Use of a cross correlation method is appropriate only for narrow band signals and also lacks a high degree of precision.